Most of the conventional fixing apparatuses for electrophotographic printing processes are of a heat roller type. The heat roller fixing apparatus includes a heat roller maintained at a predetermined temperature and a pressing roller having an elastic layer which is press-contacted to the heating roller. A transfer material, for example paper, which has an unfixed toner image on it, is passed through the nip between the heat roller and the pressing roller and heated to fix the image. In this type of image fixing apparatus, there frequently is a problem due to the so-called "toner offset phenomenon." That is, toner undesirably transfers from the paper to the heating roller. In order to prevent the toner offset phenomenon, the temperature of the heating roller has to be maintained at an optimum level. This requires a large thermal capacity for the heat roller. The large thermal capacity requires a longer period of time to increase the temperature of the heat roller to the required operating level, thereby necessitating a longer waiting time upon start of the apparatus.
In order to alleviate the above-mentioned problems, the following types of fixing apparatuses have been proposed:
1. a fixing apparatus having an electrically conductive, self-heating (resistance) film as the fixing film (see, for example, Japanese Laid Open Patent Application 3-144676); and PA1 2. a fixing apparatus heating intensively a transfer material through a fixing film (see, for example, Japanese Laid Open Patent Application 63-313182 and U.S. Pat. No. 5,493,379, Kuroda, et.al., issued Feb. 20, 1996). This latter type of device is known as a heated belt fuser. PA1 a base member having two faces and two longitudinal ends; PA1 one or more resistors extending along the length of the first face of said base member, said one or more resistors capable of generating heat upon supply of electric power thereto; PA1 first electric power supply contact for supplying electric power to said one or more resistors, said first contact located on the first face of said base member; PA1 a temperature detecting element for detecting the temperature of said base member, located on the second face of said base member; and PA1 second electric power supply contact for supplying electric power to said temperature detecting element; said second contact located on the second face of said base member. PA1 a heater, as described above; PA1 a film in slidable contact with said heater; and PA1 a back-up member cooperative with said heater to form a nip with said heater with said film therebetween; wherein a recording material (e.g., paper) carrying an unfixed toner image is nipped and moved through said nip so that the image is fixed on the recording material.
In a heated belt fuser apparatus, a heater (generally a ceramic heater) is in contact with a belt (made, for example, from a polyimide material) which moves at the same speed as the paper carrying the unfixed toner image. A pressing roller forms a nip with the heated belt through which the paper passes. As the paper passes through the nip, the heated belt fixes the toner image on the paper. Because the heater heats up quickly and also includes a temperature detecting and regulating device, which carefully controls the temperature of the heater/belt within the desired range, the heated fuser belt apparatus overcomes the problems discussed above.
The heater used in a heated belt fuser apparatus typically comprises one or more resistors, typically coupled to an AC current source, to provide the required heat, and a temperature detection and regulation device, such as a thermistor or thermostat, typically connected to a DC current source, to sense and control the overall temperature of the heater and, therefore, of the fuser belt. This temperature control keeps the temperature of the fuser belt within the desired range for fixing the toner, as well as preventing fires and scorching of the paper on which the toner image is printed. The heater is generally made from a flat ceramic material having two faces and two ends. The various components of the heater can be placed on the ceramic substrate in a variety of locations. For example, both the resistor wires and the temperature detection device can be placed on one face of the substrate. Generally, however, one component is placed on each face of the substrate (for example, the resistor wires being placed on the top (front) face, which contacts the fuser belt, and the temperature detection device being placed on the bottom (back) face of the substrate).
The electrical terminals for the resistor wires and the temperature detection device are frequently located on one face of the heater (i.e., both on the top face or both on the bottom face of the substrate). Thus, for example, U.S. Pat. No. 5,493,379, Kuroda et.al., issued Feb. 20, 1996, describes a heater for use with a fuser belt device. In FIG. 3, a heater having the heating resistor on the top face of the substrate and the temperature sensor on the bottom face of the substrate is shown. In this illustrated device, both the AC electrical terminal for the resistor and DC terminal for the temperature sensor are connected on the bottom face of the substrate. This patent teaches that if the AC power to the resistor comes in at one longitudinal end of the substrate and the DC power for the temperature sensor comes in at the opposite longitudinal end, the AC and DC sources are as far from each other as possible and this minimizes electrical noise in the DC line (which can cause erroneous readings in the temperature sensor).
This type of structure results in a number of problems for the ceramic heater. Specifically, it requires that holes be drilled through the ceramic substrate to complete the electrical connection. This weakens the substrate and increases the chance of breakage and waste during the manufacturing process and during use. Further, placement of both connections on the same side of the substrate generally requires a larger substrate, increasing the size and materials costs of the ceramic heater.
It has now been found that if the ceramic heater is formulated such that the resistor wires and the temperature detection device are placed on opposite faces of the substrate and that the electrical connections for each these components is placed on the same face of the substrate as its connected circuit lies, these problems are overcome. Thus, for example, the resistor wires may be placed on the top face of the substrate and the AC connection for the resistors is also made on the top face, while the temperature detection device is placed on the bottom face of the substrate and the DC connection for that device comes into the bottom face. This approach to formulating ceramic heaters for use in a fuser belt device means that the heater substrate can be made smaller, which lowers their cost. In addition, holes through the substrate are eliminated, making the heater easier to manufacture and minimizing substrate breakage during both manufacture and use.
Ceramic heaters are well known in the art for a variety of purposes.
U.S. Pat. No. 4,762,982, Ohno, et.al., issued Aug. 9, 1988, describes glow plugs which incorporate ceramic heaters and are used in diesel engines. The purpose of the invention is to avoid cracks in the ceramic heaters at the high temperatures required in diesel engines (approximately 900-1150.degree. C.). This is accomplished by reversing the direction of current flow in the heater during alternate usages of the heater. Figure 6 of the patent illustrates a heater which includes two DC resistor wires; all electrical terminals for the heater are located on the same side of the ceramic substrate.
U.S. Pat. No. 4,697,165, Ishiguro, et.al., issued Sep. 29, 1987, describes a heater/oxygen sensor device that is used for monitoring automobile exhaust gases. Figures 1 and 5 of the patent illustrate a device having an oxygen sensor on the top surface (with no power leads) and a ceramic heater with its power leads on the bottom surface.
U.S. Pat. No. 4,505,783, Mace, et.al., issued Mar. 19, 1985, describes an oxygen sensor device for use in an automobile which includes both temperature sensing and heating components. These components are not placed on opposite faces of the device and the power leads all come in on the same surface of the device (see Figures 1 and 2).
U.S. Pat. No. 4,883,947, Murase, et.al., issued Nov. 28, 1989, describes a ceramic heater for use in an oxygen sensing device. Figure 5 of the patent illustrates a device having an oxygen-sensing component on its top surface and ceramic heater and its connectors to a power source located on the bottom surface of the device.
U.S. Pat. No. 5,541,719, Tamaki, issued Jul. 30, 1996, describes a heated fuser belt device. The patent describes the heater used in this device in very general terms and does not speak to the placement of power connections on the device (see Figure 2 and Column 4, lines 55 et.seq.).
U.S. Pat. No. 5,444,521, Tomayuki, et.al., issued Aug. 22, 1995, describes a heated fuser belt device. The heater used in this device is described in general terms and the patent does not speak to the placement of electrical connections on the heater (see, for example, the top of column 5).